"The Bantu people who currently live there, they were the first people practising agriculture," Professor Tarduno says. "So at times of drought, you can imagine it was quite stressful on the population. And they practised, according to the archaeologists, a type of ritualistic burning. They would actually burn down their villages and then start anew."

The record they left us is proving revealing.

The shifts and swirls in the area's magnetic field have been taking shape for more than a thousand years.

"We think we see several episodes that are similar to what is happening today - that is the magnetic field is operating abnormally as viewed out of Africa," Professor Tarduno says.

On the left, the Earth’s magnetic field we’re used to. On the right, a model of what the magnetic field might be like during a reversal. NASA/Gary Glazmaier, CC BY

TWIST IN THE TALE

At the heart of the matter, geologists believe, is a disturbance in the outer core of the Earth's interior (2900km below the surface). This superheated pool of molten metal is what generates the magnetic field.

"If we look at our best numerical simulations of a magnetic field reversal, this is the type of pattern we see right before a reversal," says Professor Tarduno. "We don't know if the current (anomaly) will lead to a full reversal."

The burnt-clay huts reveal similar dips in the magnetic field happened around 400-450 BC, 700-750 BC, and 1225-1550 BC.

This mean's the current weakening has been developing for some time.

"We're getting stronger evidence that there's something unusual about the core-mantel boundary under Africa that could be having an important impact on the global magnetic field," Professor Tarduno says.

"We now know this unusual behaviour has occurred at least a couple of times before the past 160 years, and is part of a bigger long-term pattern," Dr Hare says.

"However, it's simply too early to say for certain whether this behaviour will lead to a full pole reversal."